arXiv subjects

Paul C. Hewett

Publications and source records attributed to Paul C. Hewett.

70 records · Page 4Linked to original sources

arXiv2006

The 2dF-SDSS LRG and QSO Survey: The Star Formation Histories of Luminous Red Galaxies

We present a detailed investigation into the recent star formation histories of 5,697 Luminous Red Galaxies (LRGs) based on the Hdelta (4101A) and [OII] (3727A) lines. LRGs are luminous (L>3L*), galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study we utilise LRGs from the recently completed 2dF-SDSS LRG and QSO survey (2SLAQ). Equivalent widths of the Hdelta and [OII] lines are measured and used to define three spectral types, those with only strong Hdelta absorption (k+a), those with strong [OII] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (~80 per cent), however significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low redshift comparison. While the em and em+a fractions are consistent with the low redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected life time of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted level of k+a and em+a activity is not sufficient to reconcile the predicted mass growth for massive early-types in a hierarchical merging scenario.

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arXiv2006

The UKIRT Infrared Deep Sky Survey ZYJHK Photometric System: Passbands and Synthetic Colours

The UKIRT Infrared Deep Sky Survey is a set of five surveys of complementary combinations of area, depth, and Galactic latitude, which began in 2005 May. The surveys use the UKIRT Wide Field Camera (WFCAM), which has a solid angle of 0.21deg^2. Here we introduce and characterise the ZYJHK photometric system of the camera, which covers the wavelength range 0.83-2.37 microns. We synthesise response functions for the five passbands, and compute colours in the WFCAM, SDSS and 2MASS bands, for brown dwarfs, stars, galaxies and quasars of different types. We provide a recipe for others to compute colours from their own spectra. Calculations are presented in the Vega system, and the computed offsets to the AB system are provided, as well as colour equations between WFCAM filters and the SDSS and 2MASS passbands. We highlight the opportunities presented by the new Y filter at 0.97-1.07 microns for surveys for hypothetical Y dwarfs (brown dwarfs cooler than T), and for quasars of very--high redshift, z>6.4.

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arXiv2005

Simulating Wide-Field Quasar Surveys from the Optical to Near-Infrared

A number of deep, wide-field, near-infrared surveys employing new infrared cameras on 4m-class telescopes are about to commence. These surveys have the potential to determine the fraction of luminous dust-obscured quasars that may have eluded surveys undertaken at optical wavelengths. In order to understand the new observations it is essential to make accurate predictions of surface densities and number-redshift relations for unobscured quasars in the near-infrared based on information from surveys at shorter wavelengths. The accuracy of the predictions depends critically on a number of key components. The commonly used single power-law representation for quasar SEDs is inadequate and the use of an SED incorporating the upturn in continuum flux at lambda~12000 A is essential. The presence of quasar host galaxies is particularly important over the restframe wavelength interval 8000 < lambda < 16000 A and we provide an empirical determination of the magnitude distribution of host galaxies using a low redshift sample of quasars from the SDSS DR3 quasar catalogue. A range of models for the dependence of host galaxy luminosity on quasar luminosity is investigated, along with the implications for the near-infrared surveys. Even adopting a conservative model for the behaviour of host galaxy luminosity the number counts for shallow surveys in the K-band increase by a factor of two. The degree of morphological selection applied to define candidate quasar samples in the near-infrared is found to be an important factor in determining the fraction of the quasar population included in such samples.

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arXiv2005

Calibrating Photometric Redshifts of Luminous Red Galaxies

We discuss the construction of a photometric redshift catalogue of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS), emphasizing the principal steps necessary for constructing such a catalogue -- (i) photometrically selecting the sample, (ii) measuring photometric redshifts and their error distributions, (iii) and estimating the true redshift distribution. We compare two photometric redshift algorithms for these data and find that they give comparable results. Calibrating against the SDSS and SDSS-2dF spectroscopic surveys, we find that the photometric redshift accuracy is $σ\sim 0.03$ for redshifts less than 0.55 and worsens at higher redshift ($\sim 0.06$). These errors are caused by photometric scatter, as well as systematic errors in the templates, filter curves, and photometric zeropoints. We also parametrize the photometric redshift error distribution with a sum of Gaussians, and use this model to deconvolve the errors from the measured photometric redshift distribution to estimate the true redshift distribution. We pay special attention to the stability of this deconvolution, regularizing the method with a prior on the smoothness of the true redshift distribution. The methods we develop are applicable to general photometric redshift surveys.

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arXiv2005

Evidence for dust reddening in DLAs identified through CaII H&K absorption

We present a new sample of 31 CaII(H&K) 3935,3970 absorption line systems with 0.84 99.99% confidence. The discovery of significant amounts of dust in a subsample of DLAs has direct implications for studies of the metallicity evolution of the universe and the nature of DLAs in relation to high redshift galaxies. The gas:dust ratio is discussed. Our results suggest that at least ~40% of the CaII absorption systems are excluded from the magnitude-limited SDSS quasar sample as a result of the associated extinction, a fraction similar to the upper limit deduced at higher redshifts from radio-selected surveys.

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arXiv2005

The lens and source of the optical Einstein ring gravitational lens ER 0047-2808

(Abridged) We perform a detailed analysis of the optical gravitational lens ER 0047-2808 imaged with WFPC2 on the Hubble Space Telescope. Using software specifically designed for the analysis of resolved gravitational lens systems, we focus on how the image alone can constrain the mass distribution in the lens galaxy. We find the data are of sufficient quality to strongly constrain the lens model with no a priori assumptions about the source. Using a variety of mass models, we find statistically acceptable results for elliptical isothermal-like models with an Einstein radius of 1.17''. An elliptical power-law model (Sigma \propto R^-beta) for the surface mass density favours a slope slightly steeper than isothermal with beta = 1.08 +/- 0.03. Other models including a constant M/L, pure NFW halo and (surprisingly) an isothermal sphere with external shear are ruled out by the data. We find the galaxy light profile can only be fit with a Sersic plus point source model. The resulting total M/L_B contained within the images is 4.7 h_65 +/-0.3. In addition, we find the luminous matter is aligned with the total mass distribution within a few degrees. The source, reconstructed by the software, is revealed to have two bright regions, with an unresolved component inside the caustic and a resolved component straddling a fold caustic. The angular size of the entire source is approx. 0.1'' and its (unlensed) Lyman-alpha flux is 3 x 10^-17 erg/s/cm^2.

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arXiv2005

Peering through the OH-forest: a new technique to remove residual sky features from SDSS spectra

The Sloan Digital Sky Survey (SDSS) currently provides by far the largest homogeneous sample of intermediate signal-to-noise ratio (S/N) optical spectra of galaxies and quasars. The fully automated SDSS spectroscopic reduction pipeline has provided spectra of unprecedented quality that cover the wavelength range 3800-9200A. However, in common with spectra from virtually all multi-object surveys employing fibres, there remain significant systematic residuals in many of the spectra due to the incomplete subtraction of the strong OH sky emission lines longward of 6700A. The S/N over substantial wavelength regions in many spectra is reduced by more than a factor of 2 over that expected from counting statistics. We present a method to automatically remove the sky residual signal, using a principal component analysis (PCA) which takes advantage of the correlation in the form of the sky subtraction residuals present in each spectrum. Application of the method results in spectra with essentially no evidence for degradation due to the incomplete subtraction of OH emission features. A dramatic improvement in the quality of a substantial number of spectra, particularly those of faint objects such as the bulk of the high-redshift quasars, is achieved. We make available IDL code and documentation to implement the sky residual subtraction scheme on SDSS spectra included in the public data releases. We illustrate the power of the sky-residual subtraction scheme using samples of SDSS galaxy and quasar spectra, presenting tests involving the near-infrared CaII triplet absorption, metal absorption line features in damped Lyman-alpha systems and composite spectra of high-redshift quasars.

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arXiv2003

The Frequency and Radio Properties of Broad Absorption Line Quasars

A sample of 67 Broad Absorption Line quasars (BALQSOs) from the Large Bright Quasar Survey (LBQS) is used to estimate the observed and intrinsic fraction of BAL quasars in optically--selected samples at intermediate (B_J \simeq 18.5) magnitudes. The observed BALQSO fraction in the redshift range 1.5 < z < 3.0 is 15\pm3%. A well--determined, empirical, k--correction, to allow for the differences in the spectral energy distributions of non--BALQSOs and BALQSOs shortward of \simeq 2100A in the restframe, is applied to the sample. The result is an estimate of the intrinsic fraction of BALQSOs, in the redshift range 1.5 < z < 3.0, of 22+/-4%. This value is twice that commonly cited for the occurrence of BALQSOs in optically--selected samples and the figure is in reasonable agreement with that from a preliminary analysis of the SDSS Early Data Release. The fraction of BALQSOs predicted to be present in an optical survey with flux limits equivalent to that of the FIRST Bright Quasar Survey (FBQS) is shown to be \simeq 20%. The BALQSO fractions derived from the FBQS and the LBQS suggest that optically--bright BALQSOs are half as likely as non-BALQSOs to be detectable as S_1.4GHz > 1mJy radio sources.

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arXiv2002

Spectroscopic detection of quasars in the 2dF Galaxy Redshift Survey

The 100,000 spectra from the 2 degree Field Galaxy Redshift Survey (2dFGRS) in the 100k Public Data Release represent the largest single compilation of galaxy spectra available. By virtue of its sheer size and the properties of the photometric catalogue that defines the sample, the 2dFGRS is expected to contain a number of potentially interesting objects other than galaxies. A search of the spectra in the 100k Data Release results in a census of 55 candidate high-redshift (z > 0.3) quasars. One additional 2dFGRS spectrum of a low-redshift galaxy shows an apparent anomalous broad emission feature perhaps indicating the presence of a gravitationally lensed quasar. These objects have been identified primarily using two automated routines that we have developed specifically for this task, one of which uses a matched filter and the other a wavelet transform. A number of the quasar images possess complicated morphologies, suggesting the presence of either diffuse foreground objects along the line-of-sight or very nearby point sources. The quasar catalogue will form a target list for future absorption and lensing studies, as well as providing an assessment of the loss of quasars with non-stellar images from the companion 2dF QSO Redshift Survey.

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arXiv2001

The Large Bright Quasar Survey VII. The LBQS and FIRST Surveys

The source catalogue for the LBQS and the FIRST Survey are compared in their regions of overlap. In the 270 deg^2 common to both surveys the LBQS contains \~100,000 stellar and ~40,000 non-stellar objects, while the FIRST catalogue contains ~25,000 sources. Cross-correlation of these lists yields 67 positional coincidences between known LBQS quasars and FIRST sources and an additional 19 stellar and 149 non-stellar positional coincidences with the radio catalogue. Spectroscopy of all the stellar matches and two-thirds of the non-stellar matches produces eight new quasars. One BL Lac object, previously misclassified during the LBQS survey is also identified. The straightforward fractional incompleteness of the LBQS determined from this sample is 13+/-4%, in good agreement with the published estimate of 10%. The distributions of the ratio of radio-to-optical power, apparent magnitude and spectroscopic properties for the new objects are consistent with those of the 67 LBQS-FIRST objects previously known. The consistency of the optical and radio properties of the new objects with those of the known quasars thus supports the conclusion that no new population of objects, constituting more than ~7% of quasars detected by FIRST, has eluded the LBQS optical selection techniques. The percentage of radio-detected quasars in the LBQS catalogue is found to be 12+/-2%, considerably smaller than the value of 25% advocated by White et al. (2000) based on the FBQS. Apparent differences in the form of the number-redshift relation for the LBQS and FBQS samples are shown to arise in large part from the very different optical passbands employed.

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arXiv2000

Emission Line Properties of the Large Bright Quasar Survey

We present measurements of the optical/UV emission lines for a large homogeneous sample of 993 quasars from the Large Bright Quasar Survey. Our largely automated technique accounts for continuum breaks and galactic reddening, and we perform multicomponent fits to emission line profiles, including the effects of blended iron emission, and of absorption lines both galactic and intrinsic. Here we describe the fitting algorithm and present the results of line fits to the LBQS sample, including upper limits to line equivalent widths when warranted. The distribution of measured line parameters, principally equivalent width and FWHM, are detailed for a variety of lines, including upper limits. We thus initiate a large-scale investigation of correlations between the high energy continuum and emission lines in quasars, to be extended to complementary samples using similar techniques. High quality, reproducible measurements of emission lines for uniformly selected samples will advance our understanding of active galaxies, especially in a new era of large surveys selected by a variety of complementary methods.

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arXiv1999

Limits on the star formation rates of z>2 damped Ly-alpha systems from H-alpha spectroscopy

We present the results of a long-slit K-band spectroscopic search for H-alpha emission from eight damped Ly-alpha absorbers (DLAs) at z>2 with the goal of measuring the star-formation rates in these systems. For each system we searched for compact sources of H-alpha emission within a solid angle 11x2.5arcsec^2 (44x10 kpc^2/h^2, for q_0=0.5, H_0=100 h km/s/Mpc). No H-alpha emission was detected above 3-sigma limits in the range (6.5 - 16)x1E-20 W/m^2, equivalent to star formation rates of (5.6 - 18) M_sun/yr/h^2, for a standard IMF, assuming the lines are spectrally unresolved (<650 km/s FWHM). We compare these results against the predictions of the models of Pei & Fall (1995) of the global history of star formation, under two different simplifying hypotheses: i.) the space density of DLAs at z=2.3 is equal to the space density of spiral galaxies today (implying DLA disks were larger in the past, the `large-disk' hypothesis); ii.) the sizes of DLAs at z=2.3 were the same as the gas sizes of spiral galaxies today (implying DLA disks were more common in the past, the `hierarchical' hypothesis). Compared to the previous most sensitive spectroscopic search our sample is twice as large, our limits are a factor greater than two deeper, and the solid angle surveyed is over three times as great. Despite this our results are not in conflict with either the large-disk hypothesis, because of the limited solid angle covered by the slit, or the hierarchical hypothesis, because of the limited sensitivity.

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arXiv1999

High-Redshift Gravitationally Lensed Galaxies and Tunable Filter Imaging

Most surveys for multiply-imaged gravitational lenses, outside of rich galaxy clusters, are based on sifting through large samples of distant sources to identify the rare examples of lensing. An alternative strategy, based on the selection of optimal lines-of-sight, offers a number of significant advantages. Utilising the multiplex capability of wide-area multifibre spectroscopy, together with tunable Fabry-Perot imaging, it is now possible to undertake such investigations. Progress in compiling a sample of high-redshift star-forming galaxies, gravitationally lensed by massive early-type galaxies at intermediate redshift, z~0.4, is described.

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arXiv1997

Two Close-Separation Quasar-Quasar Pairs in the Large Bright Quasar Survey

We present photometric and spectroscopic observations of two close quasar-quasar pairs found in the Large Bright Quasar Survey (LBQS) (Hewett et al. 1995, AJ, 109, 1498). The two components of the 2153-2056 pair ($z=1.845$, $Δθ=7\farcs8$, $B=17.9$ and 21.3) have the same redshifts within the relatively large uncertainty, $Δv_{A-B} =-1100\pm1500 km s^{-1}$, of the observations. The quasars are most likely spatially coincident although the possibility that the pair is the result of gravitational lensing cannot be ruled out. The two components of 1148+0055 ($z=1.879$, $B=18.5$ and $z=1.409$, $B=21.1$, $Δθ=3\farcs9$) have disjoint redshifts and the pair has attracted some attention in the context of gravitational lensing (e.g. Wampler 1997, ApJ, 476, L55) following the independent discovery of the pair by Surdej and collaborators. Four close, $Δθ\le10\farcs0$, quasar-quasar pairs have now been discovered in the LBQS and we discuss the probability of identifying pairs with disjoint redshifts and of locating spatially coincident pairs from the systematic investigation of the well-defined quasar catalogue.

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arXiv1996

ROSAT Soft X-Ray Properties of the Large Bright Quasar Survey: Modeling of Stacked X-Ray Spectra

We develop and apply a novel method of analysis to study the X-ray spectral properties of 908 QSOs in the Large Bright Quasar Survey (LBQS) that were observed during the soft X-ray ROSAT All-Sky Survey (RASS). Due to the relatively short (<600sec) RASS exposure times, only 10% of the QSOs are detected in X-rays, so X-ray spectral model fits for individual QSOs are precluded by poor photon statistics. Spectral stacking provides effectively much more sensitive X-ray observations for an average QSO in bins of redshift, and for several classes of QSOs. We model the stacked X-ray spectra in a way that obviates both the distinction between detections and non-detections, and the need to stack together only those objects observed through similar Galactic column densities N_H. In application to the LBQS/RASS sample, we marginally confirm a flattening of the X-ray spectral index of QSOs toward higher redshifts. Radio-loud QSOs show flatter X-ray photon indices, in agreement with previous work. We show that the apparent flattening of the photon index with redshift is not due to an increasing fractional contribution from the radio-loud class of QSOs.

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arXiv1996

The Radio Properties of Optically Selected Quasars. III. Comparison Between Optical and X-Ray Selected Samples

A sample of 103 quasars from the Large Bright Quasar Survey (LBQS) has been observed with the VLA at 8.4 GHz to study the evolution of the radio luminosity distribution and its dependence on absolute magnitude. Radio data from pointed observations are now available for 359 of the 1055 LBQS quasars. The radio-loud fraction is constant at ~10% over the absolute magnitude range -28 <= MB <= -23, and it rises to ~20% (log R > 1) or ~35% (log L > 25) at the brightest absolute magnitudes in the sample. This nearly flat distribution differs markedly from those of the optically selected Palomar-Green (PG) Bright Quasar Survey and the X-ray selected Extended Medium Sensitivity Survey (EMSS), both of which have lower radio-loud fractions for absolute magnitudes fainter than MB = -24 and higher fractions at brighter magnitudes. The reason for the high radio-loud fraction at bright absolute magnitudes in the PG, compared to the LBQS and other optically selected quasar surveys, is unknown. The trend of increasing radio-loud fraction with absolute magnitude in the EMSS is due at least in part to a correlation between X-ray and radio luminosity. Combining the LBQS data with radio studies of high-redshift quasars leads to the conclusion that the radio-loud fraction in optically selected quasars does not appear to evolve significantly, aside from a modest increase at z ~1, from z = 0.2 to redshifts approaching 5, a result that is contrary to previous studies which found a decrease in radio-loud fraction with increasing redshift by comparing the low-z fraction in the PG to higher redshift samples.

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